There has been conventionally known a substrate processing apparatus configured to process a substrate such as a semiconductor wafer and a glass substrate (hereinafter simply referred to as “wafer”), by immersing the wafer in a process liquid such as a deionized water and a chemical liquid. Such a substrate processing apparatus includes: a processing tank storing a process liquid, the processing tank being configured to process a plurality of, e.g., fifty wafers by immersing the wafers all together in the process liquid stored therein; and a circulation path to which the process liquid is sent from the processing tank, and through which the process liquid is returned to the processing tank.
It is preferable that the process liquid stored in the processing tank is maintained at a preset temperature, in order that a wafer can be suitably processed. Thus, the circulation path is provided with a heating unit configured to heat a process liquid flowing through the circulation path. By heating the process liquid flowing through the circulation path by the heating unit, the process liquid in the processing tank can be maintained at a preset temperature. In addition, the processing tank is provided with a temperature measuring sensor configured to measure a temperature of the process liquid stored in the processing tank. Further, the substrate processing apparatus includes a control part configured to control the heating unit based on the temperature of the process liquid which is measured by the temperature measuring sensor. The control part is configured to control the heating unit so as to adjust a heating degree of the process liquid heated by the heating unit, such that the temperature of the process liquid measured by the temperature measuring sensor can be maintained at a preset temperature.
More specifically, the heating unit includes a plurality of (e.g., four) heaters that are arranged in parallel. The control part performs an ON/OFF control of the respective heaters, so as to adjust a heating degree of the process liquid heated by the heating unit. As an ON/OFF control method of each heater by a control part, the method disclosed in the specification of JP Patent No. 3467401 is known, for example.
With reference to FIGS. 6 and 7, there is described the ON/OFF control of each heater which is disclosed in the JP Patent No. 3467401. As shown in FIG. 6, the control part is configured to control all the heaters in a periodically divided manner during each heating cycle. Herein, the control of the heaters in the periodically divided manner means a control in which the respective heaters are alternately kept on for a predetermined heater turn-on time during each heating cycle, with intervals between timings at which the respective heaters are switched on being made constant. To be more specific, as shown in FIG. 6, during each heating cycle, a heater 1 out of the four heaters is firstly switched on. Then, after a predetermined period has passed from when the heater 1 was switched on, a heater 2 is switched on. Then, after a predetermined period has passed from when the heater 2 was switched on, a heater 3 is switched on. Then, after a predetermined period has passed from when the heater 3 was switched on, a heater 4 is switched on. After being switched on, the heaters 1 to 4 are respectively kept on for a predetermined heater turn-on time. After a predetermined heater turn-on time has passed from when the heater 4 was switched on, the heater 4 is switched off. At this time, a certain heating cycle is completed, and a succeeding heating cycle is started.
When all the heaters are simultaneously kept on for a predetermined time during each heating cycle, there is a possibility that some of the heaters or all the heaters could not be simultaneously used, when the heaters come to the end of their lives because of the long use of the heating unit. On the other hand, as shown in FIG. 6, when all the heaters are controlled in the periodically divided manner during each heating cycle, the heaters are alternately kept on during each heating cycle. Thus, it can be restrained that some heaters or all the heaters cannot be simultaneously used when the heating unit is used for a long time.
When tungsten inside the heater of the heating unit is evaporated and run out, the heater experiences a breaking of wire. Upon the braking of wire, the heater cannot be used. In order to withhold the evaporation of tungsten, it is effective that a current value of the heater is increased during the use thereof, and that a temperature in a lamp of the heater is increased. Due to the increase in the temperature in the lamp of the heater, a pressure in the lamp is increased so that the evaporation of the tungsten can be restrained.
More specifically, in order to withhold the evaporation of the tungsten inside the heater, the temperature in the heater is required to be within a predetermined range, specifically, a range between 250° C. and 400° C., for example. In order that the temperature in the heater can be within the predetermined range, it is necessary that each heater turn-on time is set at a predetermined time, specifically, two seconds or more.